1. Field of the Invention
The present invention relates to an optical scanner for use in image forming apparatus typified by laser printers and copiers.
2. Description of the Related Art
Scanning lens optics in optical scanners have been proposed in various types that are composed of either one or more than one lens element. Known among these is the combination of spherical or aspheric lens optics provided near a light deflecting means and an elongated element such as an elongated cylindrical lens or mirror that has power only in a direction perpendicular to the main scanning direction and which is provided near the medium to be scanned [see, for example, Unexamined Published Japanese Patent Application (kokai) Nos. 230307/1994 and 286106/1996]. The elongated element has two functions, one for making the light deflecting means conjugated with the medium to be scanned so as to reduce the effect of errors in the angle of unintentional deviation by the light deflecting means in the direction perpendicular to the main scanning direction, and the other for reducing the magnification of the scanning lens optics in the direction perpendicular to the main scanning direction so as to suppress the variations in the size of the scanning beam spot on the medium being scanned that are caused by the lens shape and the errors in lens arrangement.
The elongated elements under consideration have no power in the main scanning direction, so they have little effect on the uniformity of the speed at which the light beam moves across the medium to be scanned.
To ensure uniformity in beam moving speed on optical scanners is becoming an increasingly difficult objective as recording media have a higher dot density. To deal with this situation, it has been proposed to improve the uniformity by using an elongated element having power in the main scanning direction (see Unexamined Published Japanese Patent Application Nos. 87123/1986, 106719/1988 and 213740/1998). The elongated elements disclosed in these patents have a deflecting action on the scanning beam irrespective of whether it is at the scan start end or at the finishing end and, hence, the scan width varies depending on the presence or absence of the elongated elements. The term xe2x80x9cdeflecting actionxe2x80x9d as used herein means the action by which the angle the emerging ray forms with the optical axis of the scanning lens optics is caused to differ from the angle the incident ray forms with the same optical axis.
FIG. 2 shows an example of the prior art. Indicated by 20 is a rotating polygonal mirror serving as a light deflecting means which is supported to rotate about a shaft in the direction indicated by arrow 25. Indicated by 21 and 22 are two lens elements as scanning lens optics; the lens element 22 is an elongated cylindrical lens having no power in the main scanning direction. Indicated by 23 is a medium to be scanned; 24 is the optical axis of the scanning lens optics; 26 is the principal ray of a light beam at the scan start end; 27 and 28 are the principal rays of a light beam within the scan region; and 29 is the principal ray of a light beam at the scan finishing end. Since the elongated cylindrical lens 22 has no deflecting action in the main scanning direction, the angle each of the rays 26-29 forms with the optical axis 24 on the entrance side satisfies the following relation with the angle on the exit side:
xcex8i=xcex8ixe2x80x2(i=26-29)xe2x80x83xe2x80x83(1)
where xcex8i is the incident angle and xcex8ixe2x80x2 is the exit angle.
Obviously, the elongated cylindrical lens 22 has little effect on the uniformity in the speed at which the light beam moves across the medium being scanned.
FIG. 3 shows another example of the prior art. Indicated by 30 is a rotating polygonal mirror serving as a light deflecting means which is supported to rotate about a shaft in the direction indicated by arrow 35. Indicated by 31 and 32 are two lens elements as scanning lens optics; the lens element 32 is an elongated cylindrical lens having power in both the main scanning direction and a direction perpendicular to it. Indicated by 33 is a medium to be scanned; 34 is the optical axis of the scanning lens optics; 36 is the principal ray of a light beam at the scan start end; 37 and 38 are the principal rays of a light beam within the scan region; and 39 is the principal ray of a light beam at the scan finishing end. Since the elongated cylindrical lens 32 has deflecting action in the main scanning direction, it generally satisfies the following relation:
xcex8ixe2x89xa0xcex8ixe2x80x2(i=36-39)xe2x80x83xe2x80x83(2)
Since the principal rays 36 and 39 are subject to the deflecting action of the elongated lens 32, the scan width varies depending upon its presence or absence.
An object, therefore, of the present invention is to provide an optical scanner that meets the requirement for higher dot density in recording media by ensuring that the uniformity in the speed at which a light beam moves across the medium to be scanned is improved without changing the scan width.
The above object of the invention can be attained by an optical scanner comprising a light deflecting means for scanning by deflecting a light beam from a light source and scanning lens optics for focusing the deflected light beam on a medium to be scanned, characterized in that said scanning optics consist of more than one lens element, the angle of the line dropped normal to the first face of the lens element positioned the closest to the medium to be scanned which is directed to the light deflecting means at the point where the principal ray of a light beam incident at the scan start end crosses said first face is generally equal to the angle of the line dropped normal to the second face of said lens element which is directed to the medium to be scanned at the point where said principal ray crosses said second face, and the angle of the line dropped normal to said first face at the point where the principal ray of a light beam incident at the scan finishing end crosses said first face is generally equal to the angle of the line dropped normal to said second face at the point where said principal ray crosses said second face.